Standing position/upright standing position
1. Field of the Invention
The present invention relates to a subject holding method and a subject holding device for holding a subject having a test surface such as an optical element in a stationary standing position with respect to a measuring system and a measuring instrument, such as an optical interference device, equipped with the subject holding device.
2. Description of Related Art
In semiconductors manufacturing techniques, it is typically to use photomasks for exposing semiconductor wafers in order to transfer a desired circuit pattern thereon. For the exposure of such a semiconductor wafer to a circuit pattern, it is quite usual to put a photomask so as to keep a mask surface in a substantially horizontal position during exposure, in other words, so as to hold the photomask in such a manner that an axis line (an ordinary line) of the mask surface is oriented in a direction of gravity. In order to transfer the circuit pattern precisely, the photomask should be carefully held for prevention of an occurrence of flexure of the mask surface due to external influence, for example, gravitational influence. Consequentially, there have been proposed a variety of techniques of maintaining surface flatness of photomasks during exposure of semiconductor wafer to a circuit pattern as known from, for example, Unexamined Japanese Patent Publication Nos. 9-306832 and 2000-223414.
On the other hand, there is a strong demand on the part of photomask manufacturer to measure optical properties, such as surface accuracy and a distribution of refractive index, of photomasks preferably without being affected by gravity.
For the purpose of preventing the photomask from being distorted by gravitational influence during the measurement of optical properties of the photomask, it is conceivably preferable that the photomask is held in an upright standing position, i.e. in a position in which a normal line of the mask surface is perpendicular to a direction of gravity. In this context, the photomask standing simply by itself is wonky and inept at measurement. It is essential for the measurement of mask surface that the photomask must be held in a stationary state under load during measurement.
It is, however, quite difficult to find an optimized load to be applied to the photomask Although it is obvious that the load must be large enough to maintain the photomask in a stationary standing position, if it is too large, the photomask encounters flexure large enough to lead to imprecise measurement of the mask surface. Heretofore, techniques for finding an optimized load for maintaining the photomask in a standing position without flexure have not yet put on a firm footing, and besides, operators have no alternative but to rely on their experience and skill in finding an optimized load It is hardly capable of meeting product diversification to determine an optimized load by such a rule of thumb acquired by skilled operators. Further, if the determination of an optimize load is improper, photomasks possibly cause surface deformation beyond the bounds of admissible production error, it can not be denied that photomask are possibly disqualified as acceptable products even while they are at acceptable surface quality levels.